1. Field of the Invention
The invention relates to siloxane suspensions and their use as binders for producing moldings based on mineral fibers, and also to the resultant fiber structures.
2. Description of the Related Art
Moldings composed of mineral fibers are composite materials composed of fibers and of a binder which links the fibers together so that the desired shape can be retained. They are widely used as materials, and in particular as insulating materials in the construction industry and in engineering. The production of products of this type has been known for a long time. The general method of producing these products is to apply the binder solution by spraying immediately after withdrawing of the mineral fiber from the melt, and molding the product in subsequent steps, and curing it in an oven. This process is mostly a fully continuous process and is therefore cost-effective. The binders used in this process usually comprise aqueous solutions of phenol-formaldehyde. condensates, which may have been modified with amino resins or with urea.
Although these mineral fiber products bonded with phenolic resin are widely used, e.g. in the construction industry for thermal insulation and solid-borne sound insulation, they also have a number of disadvantages which exclude certain uses. The main disadvantage is low heat resistance. In addition to this, relatively large amounts of formaldehyde and phenols are emitted during the production process, and when the finished product is subjected to thermal stress decomposition products of the organic resin used are liberated and cause pollution or even a hazard to health. The content of organic binder also frequently leads to some degree of combustibility or flammability, the result being non-compliance with some fire-protection requirements. This means that products produced using organic resins cannot generally be used under conditions where the insulating materials have to be non-flammable, or where they are subject to thermal stress. This is the case, for example, during use in fire-protection equipment or in household devices, such as stoves.
The proposal put forward to solve this problem is the use of low-melting thermoplastic glasses, e.g. those based on borate or phosphate, as binder, e.g. as disclosed in U.S. Pat. No. 5,591,505. A disadvantage of this binder is the fact that the binder does not crosslink, and therefore that the structure produced therewith from mineral fibers loses its mechanical strength, or indeed its shape, when exposed to heat. Silicone resins are known as binders for high-temperature-resistant products, for example electrical insulating materials based on mica. In this connection, reference may be made by way of example to DE-A 11 26 467. EP-A 342 519 also proposes using silicone resin emulsions instead of the phenolic resin solution as binders for producing structures based on mineral fiber. However, this provides only a partial solution to the problems of emissions. Firstly, these silicone resin emulsions comprise organic emulsifiers which, when exposed to heat, decompose to form undesirable gases. Secondly, the emulsions proposed also comprise liquid low-molecular-weight siloxanes, for example as disclosed in EP-A 366 133, which, either as they stand or via cleavage of alkoxy groups during the curing process cause emissions.
When moldings are produced from mineral fibers with pulverulent binders, it is also always necessary to use a specific mixture of previously prepared fiber product with binders, and it is also necessary to use high pressures during the process. In this connection, reference may be made by way of example to DE-A 27 56 503, EP-A 403 347 and DE-A 43 92 078. Use of conventional pulverulent binder is not generally possible in the context of the conventional product technology explained above applicable to structures composed of mineral fibers. The use of alkoxysilanes in a mixture with colloidal particles, e.g. silica sol, the materials known as nanocomposites, permits continuous production of moldings from mineral fibers, these being stable even at relatively high temperatures and having improved fire performance. However, this process has the major disadvantage that, in particular during the production of the mineral fiber products, the alkoxy groups of the silanes used produce relatively large amounts of alcohol by cleavage, because the silane mixtures used are composed of more than 75% by weight of alkoxy groups. These emissions pose a fire risk, and are also hazardous to health and to the environment. This requires purification of the resultant exhaust air, at considerable cost.